Apparatus and process for purifying a liquid

ABSTRACT

An improved continuous process and apparatus for treating an impure liquid to produce purified liquid, particularly, water, the process having an electrically activating a thermoelectric module to provide a first heated surface and a cooler surface; feeding the impure liquid to the first heated surface to produce vapour of the liquid; and transferring the vapour to the cooler surface to effect heat transfer to the cooler surface, the improvement being directing a minor portion of the vapour to the cooler surface to maintain the cooler surface at a temperature at or near the boiling point of the liquid; and transferring a major portion of the vapour to a condenser remote from the module to effect heat transfer and condensation of the vapour to produce the purified liquid and collecting the purified liquid from the condenser. The process is continuous in that it does not need to be intermittently stopped, or require auxiliary cooling of the module. Preferably, the heat from the condenser is transferred to pre-heat the impure liquid feed.

FIELD OF THE INVENTION

This invention relates to a process of purifying a liquid, particularlywater, using a thermoelectric module; and apparatus of use in saidprocess.

BACKGROUND TO THE INVENTION

Thermoelectric modules are small, solid state, heat pumps that cool,heat and generate power. In function, they are similar to conventionalrefrigerators in that they move heat from one area to another and, thus,create a temperature differential.

A thermoelectric module is comprised of an array of semiconductorcouples (P and N pellets) connected electrically in series and thermallyin parallel, sandwiched between metallized ceramic substrates. Inessence, if a thermoelectric module is connected to a DC power source,heat is absorbed at one end of the device to cool that end, while heatis rejected at the other end, where the temperature rises. This is knownas the Peltier Effect. By reversing the current flow, the direction ofthe heat flow is reversed.

It is known that a thermoelectric element (TEE) or module may functionas a heat pump that performs the same cooling function as Freon-basedvapor compression or absorption refrigerators. The main differencebetween a TEE device and the conventional vapor-cycle device is thatthermoelectric elements are totally solid state, while vapor-cycledevices include moving mechanical parts and require a working fluid.Also, unlike conventional vapor compressor systems, thermoelectricmodules are, most generally, miniature devices. A module typicalmeasures 2.5 cm×2.5 cm×4 mm, while the smallest sub-miniature modulesmay measure 3 mm×3 mm×2 mm. These small units are capable of reducingthe temperature to well-below water-freezing temperatures.

Thermoelectric devices are very effective when system design criteriarequires specific factors, such as high reliability, small size orcapacity, low cost, low weight, intrinsic safety for hazardouselectrical environments, and precise temperature control. Further, thesedevices are capable of refrigerating a solid or fluid object.

A bismuth telluride thermoelectric element consists of a quaternaryalloy of bismuth, tellurium, selenium and antimony—doped and processedto yield oriented polycrystalline semiconductors with anisotropicthermoelectric properties. The bismuth telluride is primarily used as asemiconductor material, heavily doped to create either an excess(n-type) or a deficiency (p-type) of electrons. A plurality of thesecouples are connected in series electrically and in parallel thermally,and integrated into modules. The modules are packaged between metallizedceramic plates to afford optimum electrical insulation and thermalconduction with high mechanical compression strength. Typical modulescontain from 3 to 127 thermocouples. Modules can also be mounted inparallel to increase the heat transfer effect or stacked in multistagecascades to achieve high differential temperatures.

These TEE devices became of practical importance only recently with thenew developments of semiconductor thermocouple materials. The practicalapplication of such modules required the development of semiconductorsthat are good conductors of electricity, but poor conductors of heat toprovide the perfect balance for TEE performance. During operation, whenan applied DC current flows through the couple, this causes heat to betransferred from one side of the TEE to the other; and, thus, creating acold heat sink side and hot heat source side. If the current isreversed, the heat is moved in the opposite direction. A single-stageTEE can achieve temperature differences of up to 70° C., or can transferheat at a rate of 125 W. To achieve greater temperature differences, i.eup to 131° C., a multistage, cascaded TEE may be utilized.

A typical application exposes the cold side of the TEE to the object orsubstance to be cooled and the hot side to a heat sink, which dissipatesthe heat to the environment. A heat exchanger with forced air or liquidmay be required.

Water in bulk may be purified by a number of commercial methods, forexample by reverse osmosis and by distillation processes.

Reverse osmosis (R.O.) technology relies on a membrane filtration systemthat is operated under high pressure. While this technology is one ofthe two leading technologies of water purification, it suffers from thefollowing main disadvantages:

(a) the infrastructure of the system is complex because of the operatingpressure, typically 8 atmospheres, required to cause the reverse osmosisprocess in the membrane;

(b) the membrane is an expensive component that needs to be replaced,frequently, depending on the salinity and the purity of the sourcewater, generally, every 4 to 6 months. Also, there is a problem ofmembrane fouling, if the quality of the source water is not withincertain bounds. The restriction on the water quality that is inputtedinto the system precludes many sources of water or would necessitate theutilization of pretreatment systems;

(c) the amount of purified water is very low when compared to the amountof water that has to be pumped into the system. Therefore, the cost ofpumping and discharging the rejected water (capital cost to install therequired facility and the energy cost to operate and maintain it) makesthis system very costly;

(d) the quality of purified water obtained by the reverse osmosisprocess is inferior to that of distilled water, in the sense that itleaves small microorganisms and any impurities that are small enough togo through the membrane. Also, as the membrane ages, the water qualitydoes not remain consistent;

(e) the system is feasible from a physical and economical point of view,for only large commercial installations. The system is not amenable foruse in household units or even in small commercial units; and

(f) energy, operating and maintenance costs are high for the R.O.system.

The main disadvantages of distillation technologies, such as themultistage flashback evaporation systems, are:

(i) relatively large capital cost needed to assemble and install thesystem;

(ii) high energy costs to perform the evaporation, provide energy andequipment for the vacuum system and the condensation in, literally,three independent subsystems;

(iii) significant corrosion problems that necessitate significantpretreatment of input water and complete replacement of plant equipmentas frequently as every three to four years;

(iv) the system, generally, needs to be installed only near large powerplants and large bodies of water; and

(v) the disadvantages listed in item (e) and (f) hereinabove.

There is, therefore, a need to provide a means for producing a purifiedliquid, particularly water, in a safe, reliable, convenient, relativelycheap manner, having low energy requirements, and which eithereliminates or reduces the aforesaid disadvantages.

Offenlegungsschrift DE 35 39 08 6A (Wagner Finish Tech Center GmbH)published May 7, 1987, describes apparatus for the purification oforganic solvents containing paint or varnish by evaporation andcondensation by use of a Peltier element which functions as both aheating and cooling element during the evaporation and condensationstages. An essential feature is the condensation of the solvent vaporsolely on the cooling element.

It is known that in addition to the production of a temperaturedifferential across the module between the ‘hot’ and ‘cold’ surfacesthat heat may be beneficially “pumped” from the cold surface to the hotsurface through the module. For example, latent heat of condensation ofa vapor on the cold surface may be captured by the cooler element andpumped to the hot side. It is also known that the heat pumped by thecold side varies linearly with the cold side temperature.

However, in the apparatus and process described in OLS DE3539086A abalanced continuous evaporation and condensation equilibrium cannot beestablished by reason that the cold side of the module absorbs thelatent heat which is then pumped to the hot element and, thus, verysignificant amounts of latent heat of the steam generated uponcondensation must be removed from the vessel or the process ‘shut down’,intermittently, for periods of time to prevent the hot elementoverheating. This is an unsatisfactory situation when continuous processconditions are desired.

Japanese Kokai JP 07 209841, published Aug. 11, 1995 to Koicki Hayashidescribes a small, low-cost and high-efficiency developer waste solutionconcentrator for use in small-scale retail stores. The concentrator isprovided with a concentrating tank divided into an evaporation tank anda condensation tank, the upper parts of these tanks being incommunication with-each other; a heat-generating/heat-absorbing sectionwhich is made up of Peltier element parts wherein a heat-generating sideis in contact with the evaporation tank and a heat-absorbing side withthe condensation tank; a replenishing pump to control the volume ofwaste solution in the evaporation tank to a liquid volume within thecertain constant range; and a cooling section to control the temperatureof the heat-generating element side to a value within a certain constantrange. The embodiments described therein effect condensation on thecooler element side of the module surface to provide purified liquid. Tomaintain the hot surface of the element at the desired temperature, acooling fan means in conjunction with heat release fins are provided.However, such heat control means results in the need for additionalphysical items and reduced electrical and thermal energy efficiency.

There is, therefore, a need for a simple, safe, convenient and reliableprocess operable under continuous conditions of purifying a liquid,particularly water.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method andapparatus for producing a purified liquid, particularly water undercontinuous conditions in a safe, convenient, reliable and relativelycheap manner by means of thermoelectric modules to generate hot and coldelements.

Accordingly, in one aspect the invention provides an improved,continuous process for treating an impure liquid to produce purifiedliquid, said process comprising electrically activating a thermoelectricmodule to provide a first heated surface and a cooler surface; feedingsaid impure liquid to said first heated surface to produce vapor of saidliquid; and transferring said vapor to said cooler surface to effectheat transfer to said cooler surface, the improvement comprising (a)directing a minor portion of said vapor adjacent to or onto said coolersurface to maintain said cooler surface at a temperature at or near tothe boiling point of said liquid; (b) directing a major portion of saidvapor to condensation means comprising a second cooler surface to effectheat transfer to said second cooler surface and condensation of saidvapor to produce said purified liquid and collecting said purifiedliquid.

In this specification and claims the term “heatable or heated surface”means a surface of said thermoelectric module which is heated when saidmodule receives an electric current or a surface in thermalcommunication with said module as to be heated thereby. The term“coolable or cooled surface” means a surface of said module which iscooled when said module receives an electric current or a surface inthermal communication with said module as to be cooled thereby.

By the term “continuous process” in this specification is meant aprocess as defined that does not need, of necessity, to beintermittently stopped or slowed, or requiring auxiliary cooling of thethermoelectric module in order to prevent overheating of the module.

The term “minor portion” means less than half of the vapour or steamgenerated by the hot element of the module, and which is a function ofthe design of the apparatus and operating conditions as to preventoverheating of the module by excessive heat transfer to the coolerelement.

Generally less than about 40% of the steam is directed to the coolerelement.

In a particularly valuable aspect, the liquid is water, and by the term“impure water” is herein meant water containing impurities such as, forexample, dissolved salts and other matter, and/or suspended particulatematter which impure water may be evaporated and concentrated withoutunwanted carry-over of such impurities.

The term “vapor” includes “steam”.

Preferably, the invention provides process for treating an impure liquidto produce purified liquid, said process comprising

(i) electrically activating a thermoelectric module to provide a heatedsurface in a first chamber and a cooler surface in a second chamber;

(ii) feeding said impure liquid to said heated surface to produce liquidvapour;

(iii) directing said liquid vapor from said first chamber to said secondchamber;

(iv) contacting a minor portion of said liquid vapor with said coolersurface to effect heat transfer to said cooler surface;

(v) cooling a major portion of said vapor in said second chamber withcondensation means comprising a second cooler surface to effect heattransfer from said vapor to said second cooler surface and condensationof said vapor to provide said purified liquid; and

(vi) collecting said purified liquid.

In a further aspect, the invention provides an improved liquid purifierfor purifying a liquid under continuous operating conditions comprisingthermoelectric module means having a first heatable surface and acoolable surface; means for contacting said impure liquid with saidheatable surface to produce vapor of said liquid; means for transferringsaid vapor to effect heat transfer to said coolable surface; means forcondensing said vapor to said purified liquid; and means for collectingsaid purified liquid; the improvement comprising (a) means for directinga minor portion of said vapor to said coolable surface to maintain saidcooler surface at a temperature near or at the boiling point of saidliquid; (b) condenser means comprising a second coolable surface meansto effect heat transfer to said second coolable surface and,consequently, condensation of a major portion of said vapor to producesaid purified liquid; (c) means for directing said major portion of saidvapor to said condenser means; and (d) means for collecting saidpurified liquid from said condenser means, wherein said thermoelectricmodule is adapted to receive an electric current to activate said moduleto heat said heatable surface and cool said coolable surface.

In prior art apparatus, the coolable surface constitutes the means forcondensing the vapor to purified liquid.

The means for directing vapor to the coolable surface may, include, forexample, merely, conduit, passage, guide or the like which allows aportion minor of the vapor to pass to the cold sink.

In one embodiment, the invention provides a liquid purifier comprising ahousing having a first chamber and a second chamber; divider meansseparating said first and second chamber one from the other; saiddivider means comprising a thermoelectric module having a heatablesurface received within the first chamber and a coolable surface withinthe second chamber; means for contacting impure liquid with saidheatable surface within said first chamber to produce vapor; firsttransfer means for directing a minor said portion of vapor to saidcoolable surface to effect heat transfer to said coolable surface tomaintain the temperature of said coolable surface at or near the boilingpoint of said liquid; condenser means having a second coolable surfacefor condensing a major portion of said vapor within said second chamberby heat transfer to produce said purifier liquid; second transfer meansfor directing said major portion of said vapor by means to saidcondenser means; and means for pre-heating said impure liquid feed bysaid heat transfer with said condenser means; and wherein saidthermoelectric module is adapted to receive an electric current toactivate said module to heat said heatable surface and cool saidcoolable surface.

Most preferably, the apparatus has a plurality of the thermoelectricmodules aligned coplanar within a divider between the chambers and/orwithin one or more walls of the chamber.

Thus, preferably, a plurality of modules are arrayed in coplanar fashionin a planar member to provide, for example, a plurality of heatablesurfaces at one, i.e. top face of the module and a plurality of coolablesurfaces on its bottom face. The aforesaid top face may constitute aninner face of an evaporation chamber and the aforesaid bottom faceconstitute the corresponding outer face of the evaporation chamber.

Thus, the essence of the present invention is to achieve a continuousevaporation and condensation equilibrium within the apparatus byremoving the majority of the thermal (latent) energy of the vapor remotefrom the coolable surface of the module. The presence of this remotesecondary condenser surface which, preferably, effects heat transfer ofat least about 60% from the steam enables satisfactory continuousremoval of excess heating power from the module. In one embodiment, a100% conversion from vapor to liquid within the same chamber containingthe module cold side can be achieved.

Maximum efficiency of the heat pump can be achieved by maintaining thetemperature of the module coolable surface essentially at the boilingpoint of the liquid, i.e. 97°-100° C. for water by means of a suitableminor portion acting on the coolable surface to effect suitable, but notexcessive heat transfer.

In a most preferred aspect, the heat transferred from the major portionof condensed vapor with the condenser is used to pre-heat the impureliquid feed, preferably from ambient temperature to a temperature of atleast 90° C. in the case of water. This preheating of the feed waterincreases the electrical utilization efficiency to over 167% and providesignificant power savings when compared to, say, 95% power utilizationefficiencies achieved with prior art conventional water purifiers,hereinbefore described.

Thus, the present invention provides in one aspect a water purificationsystem which provides the advantages of:

(a) providing both water evaporation and cooling within the same unit;

(b) being significantly energy efficient by the use amount of electricalenergy and heat transfer to perform evaporation and condensation; whichenergy utilization does not exist in any of the water purificationtechnologies known at this time;

(c) recovering all of the water inputted into the system as pure water,without having to discharge water with high concentrations of impuritiesand salt as is the case in reverse osmosis technology;

(d) portability of the system and its ability to be scaled up over avery wide range of dimensions and capacities; and wherein the capacityof the system can be increased in a modular fashion;

(e) having the ability to energize the system from a very wide varietyof power sources, such as, for example, operable throughout in theworld, including remote areas that are not even connected to an energygeneration grid; and

(f) having the ability of the system to handle any type of waterregardless of its salinity and impurities, while still producing purewater that has the same quality as distilled water, which is free fromall organic, non-organic and microbial elements.

I have found that non-insulated surfaces of vapor-receiving chambers,conduits and the like enhance condensation of the vapor to reduce theload on the module colder surface. This advantageous arrangement can beenhanced by passing the feed liquid through or around the “cold” chamberto enhance condensation external of the module cooler surface and alsopre-heat the feed water.

It is a further aspect of the present invention to provide a pluralityof multimodule units in the form of an assembly, which may be sodesigned to be of modular construction as to be built-up to any desiredoperating size.

The apparatus according to the invention may be operated oversignificant periods of time although there may be a build-up ofimpurities in the evaporation tray of the hot side surface of the modulewhich may require down-time cleaning.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be better understood, preferredembodiments will now be described by way of example only, with referenceto the accompanying drawings wherein

FIG. 1 is a diagrammatic vertical sectional view of a water purifieraccording to the invention having evaporation and condensing chamberswith natural convection water pre-heating, and showing, in part, anenlarged thermoelectric module as an insert;

FIG. 2 is a diagrammatic vertical sectional view of a water purifieraccording to the invention having evaporation and condensing chamberswith natural and forced air convection water pre-heating;

FIG. 3 is a diagrammatic vertical sectional view of a water purifieraccording to the invention having evaporation and condensing chamberswith natural convection water pre-heating and condensing surface heattransfer boost via thermoelectric elements;

FIG. 4 is a diagrammatic vertical sectional view of a water purifieraccording to the invention having evaporation and condensing chamberswith convention and forced air convection water pre-heating andcondensing surface heat transfer boost via thermoelectric elements;

FIG. 5 is a diagrammatic vertical sectional view of a water purifieraccording to the invention having evaporation and condensing chamberswith heat exchanger coil water pre-heating; wherein the arrows indicatesteam current flows or where indicated air flows; and the same numeralsdenote like parts.

DETAILED DESCRIPTION OF THE INVENTION

Generally, a water holding compartment is used to hold water to bepurified and located on top of the heat extraction chamber so that someof an heat extracted from the condensing process is used to pre-heat thewater to be purified. This may also release some of the volatile organiccompounds present in the water. The pre-heating process reduces theamount of additional heat energy required to make the water boil in theevaporation tray.

In preferred embodiments, heat exchanger means is used to cool the topof a condensing chamber, whereby the rate of heat extraction from thetop of the condensing chamber determines the rate of condensing of thesteam produced inside the condensing chamber. Cooling is performed, forexample, by using natural convection, forced air convection with fansblowing ambient temperature air through the fins, or liquid cooling byfeeding impure feed fluid circulated through a remote secondary heatexchanger.

With reference to FIG. 1, this shows generally as 10, a closedcylindrical tank 12, formed of a plastics material and having a purifiedwater outlet 14 at tank bottom 16 and a thin thermally conductivestainless top 18. Centrally suspended by means not shown within tank 12is a cylindrical open-topped metal tank 20. The base 22 of tank 20 isconstituted by a plurality of thermoelectric modules 24 (Polar TEC™model HT4-12-30-Melcor Corporation, Trenton, N.J., U.S.A.) in coplanararray one adjacent another (twelve modules in the embodiment shown)having ceramic upper and lower surfaces 26 and 28, respectively. Uppersurface 26 constitutes the hot or heatable surface, while the lowersurface 28 constitutes, the cooler or coolable surface. The lower wallportion 30 of a hood 31 and upper surface 26 form a water evaporationtray 32. Hood 31 is suspended by attachment to an impure feed water pipe34 which passes through tank top 18 into a feed water holding tank 36,which holds water to a level 38 to the top of pipe 34. Water is fed totank 36 on demand from conduit 40 to preheat the impure liquid feed,preferably from ambient temperature to a temperature of at least 90° C.in the case of water.

Hood 31 surrounds tank 20 at a distance to define a cylindrical cavityor steam guide 42 and terminates at its lower end as an inwardlydirected lip 44 or cusp adjacent base 14. Lip 44 is so shaped as todirect a minor portion of generated steam out of guide 42 adjacent oronto module coolable surface 28.

In operation, feed water from tank 36 drops down pipe 34 in controllablefashion into tray 32 onto surface 26 whereby it is converted into steam,which rises and passes through guide 42. A portion of the steam isdirected onto surface 28 to effect heat transfer and provide theadditional heat to be pumped to hot surface 26, at a rate to maintainthe cooler surface between 97° and 100° C. The remaining major portionof the steam rises to condense on the inner surface 46 of top 18 wherebythe latent heat of condensation is transferred through top 18 topre-heat the water in tank 36, to, preferably, a temperature of at least90° C. Water produced at surface 46 runs down the side of tank 12 and isdrawn out of pipe 14.

This preheating of the feed water can increase the electricalutilization efficiency to over 167% to provide significant power savingswhen compared to, say, 95% power utilization efficiencies achieved withprior art conventional water purifiers hereinbefore described.

The power supply (not shown) consists of a high voltage and a lowvoltage section. Input power is normally derived from a 120/230VAC inputline. Alternative embodiments may use power supplied from natural energysources, such as solar or wind power. The input power is converted bythe power supply into a source of direct current at a high voltageaverage level of approximately 160V. This direct current is applied tothe thermoelectric heat pumps in the evaporation tray, as well as anythermoelectric heat pumps, associated with the impure water feed aspre-heaters, heaters or chillers in the purified water storagecompartment.

Input power is also converted by the power supply into a source ofdirect current as low voltage levels of approximately 12 and 5 volts.This low voltage power source may be used by any process supervisorunit, electronic flow control valves, feedback sensors for temperature,water level, and pressure, and by any user interface.

With reference to FIG. 2, this shows a modified version of the apparatusshown in FIG. 1 wherein top 18 is cooled by and, thus, preheats an airflow pulled into an air chamber 50 of an upper cylindrical tank 52having lower intake and upper exhaust apertures 54, 56, respectively.

Downpipe 34 communicates with water holding tank 36 and, within chamber50, a water pre-fill tray 58 via a three way valve 60. Tray 58 has upperand lower water level detectors 62 and 64, respectively, and isintermittently replenished from tank 36. An air circulation fan 66provides intake and exhaust air flow directions. Hot air generated byheat exchange surface 46 at top 18 pre-heats the water held in bothtanks 58 and 36. The remaining construction and process operation is asdescribed for the apparatus of FIG. 1.

FIG. 3 shows a modified version of FIG. 1, wherein top 18 has aplurality of thermoelectric modules 68 embedded therein or, optionally,adjacent thereto, with the coolable element 70 receiving the latent heatof condensation which is pumped to hot side 72 of modules 68. Thus, thisheat exchanger means provides pre-heating of feed impure water prior totransfer to tray 32 down pipe 34. Electrical power supply leads tomodules 68 are omitted for clarity.

FIG. 4 shows a modified version of the combined apparatus of FIGS. 2 and3, wherein forced air is pre-heated by the hot sides 72 of modules 68.

FIG. 5 shows a modified version of FIG. 1, wherein pipe 34 comprises aglass or steel heat exchanger coil 74 deposed within tank 12, wherebyimpure water feed is pre-heated.

DC power is supplied to the thermoelectric module array in bottom 24from, optionally, a solar panel 80 and/or 12 volt DC power supply 82through microprocessor control module 84.

COMPARATIVE TESTS

A high temperature, high throughput thermoelectric module (Part #HT4012-39) is commercially available from MELCOR THERMOELECTRICS.Operation of this particular module at 3.5 amperes and 15.71 volts,provides a hot side temperature of 103° C. The hot side temperature waschosen to be 103° C. in order to show the typical performance of thismodule when used to evaporate, at one atmosphere, a layer of water thatis in contact with the hot side of the module.

It is known that the heat pumped by the module cold side varies linearlywith the cold side temperature. For an electrical input of 55 W (15.71V,3.5A), the module pumps 37 Watts at a “cold” plate temperature of 97°C., 29 Watts at a cold plate temperature of 80° C., and 10 Watts at acold plate temperature of 34° C. Hence, for the same electrical inputpower, the heat pumping efficiency is 67% (37/55), 53%, and 18% when thecold side temperature is 97°, 80° and 34° C., respectively. This showsthat to utilize the maximum heat pumping capability of such a module,the cold plate temperature must be elevated to be close to that of thehot plate temperature. For example, if a layer of water is in contactwith the hot side of the module, and the cold side of the module ismaintained at a temperature of 97° C., then a total heating power of 55W electrical input plus 37 Watts of pumping equals 92 Watts is appliedthrough the hot side of the module to the water. As the water boils, anequilibrium is established where the latent heat in the steam risingfrom the surface of the water equals the heat energy being appliedthrough the hot side of the module to the water. The surface temperatureof the water equals the temperature of the steam produced, which equals100° C. for operation at a pressure of one atmosphere. Hence, 92 Wattsapplied to the water through the hot side of the module produces steamat 100° C. that has 92 Watts of latent heat. The surface temperature ofthe hot side of the module rises to higher than 100° C. since additionalheat is required to overcome the pressure produced by the weight of thelayer of water on the hot side of the module. Experiments conducted withthese modules show that in practice, the hot side temperature rises 3°C. for every 1 cm of water thickness on the module hot side tray.

In conclusion, the net effect of the module, when used to heat water, isto produce an added heating power. Hence, for an electrical input of 55Watts, a heating power of 92 Watts is produced if the cold side is at97° C., 84 Watts if the cold side is at 80° C., and 65 Watts if the coldside is at 34° C. In this embodiment, the best utilization of themodules is when the cold side temperature is at 97° C. to effectivelyproduce 1.67 Watts of water heating power for every 1 Watt of electricalinput power applied.

Prior Art OLS DE3539086A1 uses the cold side of the modules to condensethe evaporated steam that is produced by the heat transferred from thehot side of the modules into the liquid that is evaporated. If theaforesaid thermoelectric module (Part # HT4-12-30) is used in theapparatus and process described and illustrated in FIGS. 1, 2 and 3 ofDE3539086A1, then it can be reasonably assumed for comparison purposesthat the hot side temperature of the module will eventually reach 103°C., if (a) the solvent solution to be purified is water, (b) thepressure is one atmosphere, (c) the voltage applied to the module is15.71V, (d) the current consumed by the module is 3.5 A, (e) the hotside of the module is in direct contact with the solution to bepurified, and (f) the solution to be purified forms a 1 cm thick layerover the tope of the hot side of the module. As electricity is firstapplied to the module used in this embodiment, the temperatures of thehot side and cold side of the module are equal and assumed to be atambient air temperature. The hot side receives 55 Watts of heatingpower, plus whatever heating power is pumped by the cold side to the hotside of the module. As there is no temperature difference between thehot and cold sides of the module during startup, the heating powerpumped from the cold side to the hot side is at least 37 Watts. Hence,at least 92 Watts of heating power are initially applied by the hot sideof the module to the water to be purified. The water temperatureincreases to 100° C. as the heating power transferred to the water fromthe hot side of the module increases the heat energy of the water. Asthe temperature of the water increases to 100° C., so does thetemperature of the hot side of the module which is in direct thermalcontact with the water. In order to maintain the maximum level of heatpumping at a zero temperature differential between the hot and coldsides of the module, the rise in the cold side temperature must directlymatch the rise in the hot side temperature of the module. As soon as thecold side temperature rises above ambient temperature, no heat pumpingoccurs as heat can not transfer naturally from the colder ambient air tothe ‘cold’ plate. Instead, the cold side temperature will decrease belowthe ambient air temperature in order to maintain the Peltier heatpumping effect. Hence, the temperature differential between the hot andcold sides begins to increase, and at the same time the amount of heatpumping begins to decrease. This process continues until a large enoughtemperature differential between the hot and cold sides is produced sothat the natural transfer of heat between the air at ambient temperatureand the colder temperature colder side equals that rate of heat pumpingfrom the cold side to the hot side of the module.

The temperature of the water to be purified and the hot side of themodule will eventually reach 100° C. In fact, the hot side temperaturewill reach 103° C. The heating power applied by the hot side can nolonger be absorbed by the water as liquid heat energy. The water beginsto convert into steam at an equilibrium rate so that the latent heat inthe steam rising from the surface of the water equals the heat energybeing applied through the hot side of the module to the water. Thus, thelatent heating power stored in the steam rising from the surface of thewater is equal to 55 Watts, plus whatever heating power is being pumpedby the Peltier effect from the cold side to the hot side of the module.This pumped heating power, as previously stated can vary from 10 Wattsif the cold side temperature is at 34° C., to 37 Watts if the cold sidetemperature is at 97° C. At startup, the ambient air temperature insidethe apparatus described in DE3539086A1 could reasonably be assumed to be23° C. Hence, minimal added heat pumping occurs during the start of theevaporation of the water to be purified. As more steam is generated, theambient air temperature increases, and the natural rate of heat transferfrom the ambient air to the cold side of the module increases. The coldside warms up until the increased heat pumping produced by the decreasedtemperature differential between the hot and cold sides again matchesthe natural rate of heat transfer from the ambient air to the cold side.Hence, a higher heat rate and heat pumping efficiency is produced.Eventually, the temperature inside the vessel described in DE3539086A1will rise towards the 100° C. temperature of the steam being produced.Steam coming in contact with the cold side releases its latent heat andcondenses on the surface of the cold side. Equilibrium can only beestablished if the rate of condensation of the steam is equal to therate of production of the steam. In order for all of the steam that isproduced by the heating power transferred from the hot side to condenseon the cold side, the cold side must absorb 55 W, plus whatever heatpumping occurs at the cold side. Since the cold side of the module canonly absorb the heat that is pumped, 55 W of power that is stored in thelatent heat of the steam must be removed from the vessel. Accordingly, acontinuous evaporation and condensing equilibrium cannot be establishedwith the apparatus and process described in DE3539086A1.

The only way to practically implement the process described inDE3539086A1 is to follow the teaching of the present invention and toadd an external heat exchanger means to remove the excess 55 W ofheating power stored as latent heat from the steam produced from thewater to be purified. At a near optimal module cold side operatingtemperature of 97° C. and an ambient temperature of 23° C., the processdescribed in DE3539086A1 could only condense 40% {37/(55+37)} of thesteam it produces. As excess steam is produced, the pressure inside thevessel will increase until a pressure relief valve, if present, isactivated. Excess steam will then be released into the environment at arate that equals 1.5 times the rate that steam is being condensed.Hence, 60% of the purified water is lost to the environment as steam,which translates into 60% of the heating energy applied through the hotside of the module. Evaporation occurs readily as the water to bepurified is quickly converted into steam. However, the low rate ofcondensing causes a rapid pressure buildup inside the evaporationchamber, which results in both excess pressure and steam release intothe environment. Cascading of modules to increase the temperaturedifferential between the hot and cold sides only decreases the heatpumping capability of the modules, and further reduce the condensingrate of the steam.

If the goal is to purify water using the embodiment described inDE3539086A1, then 55 W of applied electrical power yields only 37 W ofenergy transfer, for an electrical power utilization efficiency rate of67%. The conventional purification of water using a heating element,such as that in a kettle, and a cooled condensing coil provides anelectrical power utilization efficiency of over 95%. In fact, a similaranalysis can be conducted to indicate that the embodiment described inDE3539086A1 is impractical for continuous use due to its inefficiency inany process that attempts to recover a liquid from a solvent solution byfirst evaporating the liquid and then condensing its vapour. If the goalis, however, to recover a higher boiling point liquid from a solventsolution by first evaporating all of the lower boiling point impurities,then the embodiment described in DE3539086A1 is practical, as itprovides, at near optimal operation, a heating power capability of 92 Wfor an electrical input power of 55 W. Hence, an electrical powerutilization efficiency of over 167% can be achieved.

In sharp contrast, the apparatus and process according to the presentinvention achieves a continuous evaporation and condensing equilibriuminside the vessel by removing the excess latent heat from the steam via“external” heat exchanger means remote from the module cooler surface.For the identical module described hereinbefore, and implemented withall of the previously made assumptions, an excess heating power of 55 Wis removed from the vessel without the actual removal of steam. Asecondary condensing surface remote from the module is utilized on whichthe remaining approximately at least 60% of the steam condenses and becollected with the no more than 40% of steam that condenses on the coldside of the module. A 100% conversion of steam to condensate can occurinside the vessel. An electrical power utilization efficiency of over167% can be achieved as 55 W of electrical input power produces theheating effect of 92 W (55 W+37 W). The 55 W of heat removed from thesecondary condensing surface is most preferably reapplied to the feedwater entering the vessel. The practical utility of the presentinvention is that it can achieve full evaporation and condensationequilibrium in the vessel, and thereby increase the throughput of theembodiment described in DE3539086A1 by, say, 250%, (i.e. 100% condensaterecovery vs. 40% condensate recovery). Pre-heating of the incoming waterto be purified also increases electrical utilization efficiency to over167% to provide significant power savings when compared with the 95%power utilization efficiency achieved with conventional water purifiers.

Although this disclosure has described and illustrated certain preferredembodiments of the invention, it is to be understood that the inventionis not restricted to those particular embodiments. Rather, the inventionincludes all embodiments which are functional or mechanical equivalenceof the specific embodiments and features that have been described andillustrated.

What is claimed is:
 1. A liquid purifier comprising a housing having afirst chamber and a second chamber; divider means separating the firstand the second chambers one from the other; the divider means comprisinga thermoelectric module having a heatable surface received within thefirst chamber and a coolable surface received within the second chamber;means for contacting impure liquid feed with the heatable surface withinthe fist chamber to produce vapor; first transfer means comprising vaporguide means for directing a minor portion of the vapor adjacent to oronto the coolable surface to effect heat transfer to the coolablesurface to maintain temperature of the coolable surface at the boilingpoint of the liquid; condenser means having a second coolable surfacefor condensing a major portion of the vapor within the second chamber byheat transfer to produce purified liquid; second transfer means fortransferring the major portion of the vapor to the condenser means;means for pre-heating the impure liquid feed by the heat transfer withthe condenser means; and the thermoelectric module comprises means forreceiving an electric current to activate the module to heat theheatable surface and cool tie coolable surface.
 2. In a continuousprocess for treating an impure liquid feed to produce purified liquid,said process comprising the steps of providing means for receivingelectricity to electrically activate a thermoelectric module to providea first heated surface and a first cooler surface; feeding said impureliquid feed to said first heated surface to produce vapor of saidliquid; and transferring said vapor to said first cooler surface, toeffect heat transfer to said first cooler surface, the improvementcomprising the steps of (a) providing vapor guide means for directing aminor portion of said vapor adjacent to or onto said first coolersurface to maintain said first cooler surface at a temperatureessentially at the boiling point of said liquid; (b) directing a majorportion of said vapor to condensation means comprising a second coolersurface to effect heat transfer to said second cooler surface andcondensation of said vapor to produce said purified liquid; (c)pre-heating said impure liquid feed by said heat transfer with saidcondensation means; and (d) collecting said purified liquid.
 3. Aprocess as defined in claim 2 wherein said cooler surface is maintainedat a temperature at which said minor portion of said vapor does notessentially condense.
 4. A process as defined in claim 3 wherein saidliquid is water.
 5. A process as defined in claim 2 wherein said liquidis water and said first cooler surface is at a temperature selected from97°-100° C.
 6. A process as defined in claim 2 wherein said liquid iswater.